The minimum resolvable structure width of an exposure tool projection system for transferring a pattern structured on a mask onto a semiconductor wafer being coated with a resist given by the term 0.25·λ/NA can theoretically be achieved by employing a full set of litho-enhancement techniques. In the formula, λ is the wavelength of the illuminating light and NA is the numerical aperture of the pupil plane, or the object lens system, respectively. The coefficient k1=0.25 is particularly challenging, and the techniques are either not yet appropriately matured or operate only under restricted conditions, e.g., for certain patterns on the mask.
Typical exposure tools operate with k1=0.4 for simple periodic lines-and-spaces patterns. A most promising candidate for imaging down the minimum structure width to 0.3·λ/NA derives from the use of alternating phase-shift masks. While not yet in a production status, this kind of mask enhances the resolution capability of a projection system in combination with the lines-and-spaces patterns. E.g., chrome lines are alternatiingly separated by spaces having two opposite degrees of phase-shift, which is exerted on the light, that traverses the mask to expose the wafer. The alternating degree of phase-shift considerably enhances the amount of structure contrast.
While it is a straightforward approach to apply alternating phase-shifts to transparent portions of the patterns, i.e., spaces, in a one-dimensional grid of structures, i.e., lines and spaces, an application in two-dimensional periodic structures is more complicated. For example, a dense structure matrix of contact holes necessitates, in the case of alternating phase-shift masks, there are always two neighboring contact holes, which in being transparent both comprise the feature of the same phase-shift. The assignment of two phases allows either to get maximum contrast in y-direction or a maximum contrast in x-direction, but no simultaneous improvement of the contrast is possible.
The use of attenuated phase-shift masks, or half-tone masks, is already matured, and is used in manufacturing of mask mass products. Attenuated phase-shift masks do not have the resolution capability for exposure tools as in the case of alternating phase-shift masks, or their chromeless supplements. The practice of manufacturing all kinds of phase-shift masks stated above is well-known to ordinary persons skilled in the art, and will not be described further herein.